Power inverters are commonly used in the electronics industry. An inverter's output terminal(s) typically are connected resistively or capacitively to ground or a grounded inverter case. Depending on the inverter's output voltage, frequency and amount of resistive and or capacitive coupling, a significant amount of high frequency current can flow between the inverter's output terminal(s) and ground. This can cause an electrical shock or a fire. The present invention prevents or limits the amount of current that can flow between a power inverter output terminal(s) and ground.
An off-line power inverter block diagram is shown in FIG. 1. 1 is an input section for AC power. 2 is a power line protection circuitry section which includes a fuse, etc. Conducted electro-magnetic interference (EMI) suppression circuitry is shown in the section 3. Here, L1 is a common mode inductor, L2a and L2b are differential mode inductors, Y1 and Y2 are equal value capacitors for limiting leakage current to earth ground where the values of Y are such the high frequency EMI generated by the inverter are shunted to earth ground while the lower frequency AC signal is not so shunted.
Section 4 is half or full wave rectification circuitry. Ripple voltage is filtered in section 5. The DC voltage is converted to a high frequency by a high frequency inverter and control circuitry. This is shown in the section 6. Section 7 is optional which is a high frequency transformer for voltage isolation, step-up, step-down or for multiple output secondary voltages.
In operation, potential difference between the neutral (N) AC line and the earth ground (EG) is very low. However, the potential difference between the live (L) AC line and the earth ground (EG) is the full AC voltage. For 120V AC input and for a full wave rectification case as illustrated in FIG. 2A, the voltage between the (+) lead and the earth ground, and voltage between the (-) lead and the earth ground are shown in FIGS. 2B and 2C, respectively.
High frequency power inverter circuitry 6 is often designed using commonly available switch-mode integrated circuits. For example, a block diagram of an off-line resonant inverter utilizing the integrated circuit (IC), SG2525 is shown in FIG. 3 and is indicated at 20. The combination of CT2 and RT2 determines the oscillator frequency of the IC. A resistor R4 is usually required between the terminals 15 and 13. The + Low Voltage connected to pin 15 may be derived from P (+) or an independent voltage source. A resistor divider R5 and R6 determines the amount of DC voltage applied to non-inverted terminal (pin 2) of an operational amplifier. This voltage, in turn, sets the magnitude of the duty cycle of the output pulses (pin 14 and pin 11). Depending on the requirements, an impedance Z2 is necessary between the inverted terminal (pin 1) and the compensation terminal (pin 9) of an error amplifier for loop stability of the IC.
Output signals from pin 11 and pin 14 periodically turn transistors Q2 and Q3 on and off. Thus, when Q2 is on, Q3 is off, and when Q2 is off, Q3 is on. During the time when Q2 is on, energy flows through Q2 and the resonant inductor LR to charge the resonant capacitor CR. Then, when Q2 is off but Q3 is on, stored energy from CR flows back through LR and Q3. Under this arrangement, if the pulse repetition frequency of the pulses at pins 11 and 14 is identical with the resonance frequency of the LC (LR and CR) network, then the circuit functions as a resonant inverter.
An off-line high frequency power inverter can also be constructed using power inverter topology other than resonant inverter topology. For example, push-pull topology, half bridge topology, etc.
The resonant inverter can drive a load like a fluorescent lamp as described by U.S. Patent application Ser. No. 147,574 filed Jan. 19, 1988 by Fazle S. Quazi which is assigned to the assignee of the subject application and which is incorporated herein by reference. During installation or removal of a lamp, a person can accidentally be in contact with terminal A and the grounded inverter case simultaneously where the fluorescent lamp would be out of contact with contact A as indicated in FIG. 3. In this situation, the person can be replaced by an equivalent resistor Rp of 500 ohms. According to Underwriters Laboratories Inc., USA, safety standards UL 935, Paragraph 20.5, when a 500 ohms resistor is placed between the terminal A and ground, the maximum acceptable peak current through the 500 ohms resistor must be limited to 43.45 milliamperes, when the inverter frequency is 10,000 Hertz or more. This corresponds to a maximum peak voltage of 21.7 volts across the 500 ohm resistor.
If the voltage across the person rises above 21.7 volts, this may be dangerous and thus, in accordance with the invention, means are provided for detecting this voltage and for taking appropriate steps when the voltage rises to 21.7 volts or any other voltage at which appropriate action should be taken.
Returning to FIG. 3, when a resistor Rp is placed between the inverter output terminal A or B and ground, current will flow between them. The magnitude of current flow will depend primarily on the following:
(a) with respect to ground, the magnitude of the high frequency AC voltage that appears at the terminal A or B; PA1 (b) the value of the resistor (Rp) placed between A or B and ground; PA1 (c) the amount of parasitic capacitive coupling (Cp) between high-frequency circuitry, wires and grounded case, as discussed below and illustrated in FIG. 4; and PA1 (d) the values of the Y capacitors used in the EMI section 3.
In the case of a transformer isolated load, the current flow will be determined by the construction of the transformer. For example, electrostatic shielding between primary and secondary winding, capacitive coupling (Cw) between these two windings, etc., will play major roles.
Possible current flow paths between the terminals A or B and the ground G are shown in FIGS. 4 and 6 where FIG. 6 includes an isolation transformer placed between the inverter output and the load. FIGS. 5 and 7 are simplified schematics of FIGS. 4 and 6, respectively. In these simplified cases, the effects of bridge rectifier diode drop voltages are neglected.
With resistor Rp in place, the high frequency voltage developed across A and G or B and G is also reflected between the points G and P (+) or between G and P (-) where P (-) is circuit ground. Thus, by placing a sensing circuit between G and P (+) or, G and P (-), the amount of current flow through Rp can be detected.